1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copier, a printer, and a facsimile, and particularly to a wet-type image forming apparatus using a liquid developer to form a toner image.
2. Description of the Related Art
In an electrophotographic image forming apparatus, toner supplied from a developing device is used to develop an electrostatic image on a photoconductor. Then, the developed toner image is transferred onto recording paper, whereby an image is formed on the recording paper. In the transfer process in such an image forming apparatus, an electrostatic transfer technique is generally adopted.
When a toner image is transferred onto recording paper serving as a transfer target, voltage is applied by a transfer roller from the back side of the recording paper conveyed to a position opposing the photoconductor to form an electric field between the photoconductor and the recording paper. The electric field causes the toner image to be electrostatically adsorbed on the recording paper.
Thereafter, the toner image formed on the recording paper is heated and pressed by a fixing device, whereby the transferred toner image is fixed on the recording paper.
On the other hand, in recent years, in an image forming apparatus such as an office printer for bulk printing and an on-demand printer that requires higher image quality and higher resolution, a wet-type developing device is known, which uses a liquid developer with a small toner particle size that is less likely to produce toner image noise. The liquid developer is prepared by dispersing toner in a carrier liquid such as a paraffin-based solvent. In the development and transfer process, toner is moved by the effect of an electric field in a toner layer containing the carrier liquid and the toner, whereby an image is transferred onto recording paper.
In this way, the carrier liquid plays an important role in motion of toner but makes the fusibility of toner lower than when using toner alone. Therefore, it is necessary to set the fixing temperature higher.
However, when the fixing temperature is set high, if the amount of carrier liquid in the toner layer is small, the toner in contact with the fixing roller easily attaches to the surface of the roller. This phenomenon is called offset. Conversely, if the amount of carrier liquid in the toner layer is too large, the toner is not sufficiently fused, resulting in poor fixing. Therefore, in order to achieve both prevention of offset and good fixing performance, it is important to adjust the amount of carrier liquid in the toner layer within an appropriate range.
In this respect, Japanese Laid-Open Patent Publication No. 09-244517 discloses a technique for maintaining fixing performance by providing a pre-fixing unit before main fixing and adjusting the set temperature, etc. in the pre-fixing unit according to a paper type.
The technique disclosed in the publication above only adjusts the set temperature, etc. according to a paper type. However, the amount of carrier liquid in the toner layer greatly varies until fixing is completed, because the carrier liquid is infiltrated from the toner layer into the recording paper after transfer onto the recording paper. Therefore, it is difficult to control the amount of carrier liquid within an appropriate range unless the amount of carrier liquid in the toner layer is grasped before fixing. It is hard to accurately control the amount of carrier liquid in the toner layer only by adjusting the set temperature, etc. according to a paper type.